The present invention provides a new process for purifying one or more organic isocyanates or isocyanate mixtures. In this process, the isocyanate material to be purified is combined with one or more alcohols and/or thiols or mixtures of alcohols and/or thiols, the resultant mixture is heated and, optionally, simultaneously with or subsequent to being heated, degassed and/or worked up by distillation and/or extraction.
Impurities of varying types and amounts are generally present in organic isocyanates due to the method used for their preparation. These impurities are the cause of variable activity. Variable activity adversely affects the reproducibility of product properties and thus the economic viability of using the isocyanate having variable activity. Both aromatic isocyanates (for example the well-known phosgenation products of aniline/formaldehyde condensation and 2,4- and 2,6-diisocyanatotoluene) and aliphatic isocyanates (such as isophorone diisocyanate) contain a whole range of impurities of this type. These impurities are mainly chlorine-containing compounds which cause variations in the activity if the chlorine is xe2x80x9chighly mobilexe2x80x9d (i.e., so-called xe2x80x9chydrolyzablexe2x80x9d) chlorine. Some of these compounds have been shown to be relatively stable and they remain in the isocyanate even after distillation. They also exert a harmful effect on the stability of an isocyanate as well as on its activity. A more uniform, smaller proportion of these contaminants, resulting in activity standardization and simpler subsequent processing of isocyanates is therefore of both technical and economic importance.
There have been many attempts to find opportunities for removing the chlorine-containing compounds from isocyanates. Additives based on metals or alkali metals such as metal oxides, metal cyanamides, metal hydrides, metal fatty acid esters in the presence of sterically hindered phenols, metal naphthenates, metal silicates, alkali metal carbonates and organometallic compounds are described in JP 4 501 032 9 B; JP 4 200 413 7 B; JP 5 908 845 2 A; JP 5 910 875 3 A; JP 5 917 245 0 A; U.S. Pat. No. 3,373,182; GB-A 1 111 581; U.S. Pat. No. 3,759,971; U.S. Pat. No. 4,094,894; ZA 8 100 606; DE-A 11 38 040; DE-A 12 86 025; U.S. Pat. No. 3,458,558; U.S. Pat. No. 3,264,336; SU 8 066 77 and DE-A 22 10 607. In each of the processes disclosed in the patents and published applications mentioned above, a number of engineering difficulties which relate to isolation of the metal-containing additives or the restricted use of metal-containing isocyanates and/or distillation residues are encountered.
Similar difficulties are encountered when using additives such as the imidazole described in GB-A 1 347 647 and JP 0 505 898 2 A; the sulfonic acids and their esters described in GB-A 1 458 747; the diethyl sulfate described in GB 1 459 691 and the sulfuric acid also described there; the trialkyl phosphate described in DD 288 596; and the use of other additives such as epoxy compounds (DE-A 22 49 375; JP 0 932 396 8 A2), tetra-substituted ureas (DD 288 598), formic or acetic acid or their derivatives (U.S. Pat. No. 3,799,963) or the compounds which contain trimethylsilyl groups described in EP-A 524 507.
Some compounds with at least one Zerewitinoff-active NH group, such as urea (DD 285 594), biurets (DD 288 597), caprolactam (DD 285 593), ammonium salts (DD 288 594), carbodiimides (DD 288 599), primary and secondary amine salts (DD 288 593), tertiary alcohols and tertiary alkyl carbamates (DD 288 595) are recommended in the prior art for purifying isocyanates. Here again, isolation of the additives or the restricted use of additive-containing isocyanates and/or distillation residues, and in particular the sometimes large decrease in NCO index and the increase in viscosity, which can be attributed to the production of biurets when using tertiary alcohols, are disadvantages. The latter also applies to the use of water for purifying isocyanates (DE-A 12 40 849).
From publications JP 6 116 125 0 A; JP 0 516 323 1 A; DE-A 19 50 101; DE-A 19 38 384; DE-A 25 32 722; DE-A 26 31 168; U.S. Pat. No. 3,853,936; FR-A 1 555 517; DE-A 29 33 601; and U.S. Pat. No. 3,549,504, it is known that isocyanates can be purified by specific distillation and crystallization techniques.
It is also known that heating isocyanates, particularly when simultaneously stripping with an inert gas, or heating in an inert solvent under pressure with simultaneous removal by suction of the volatile compounds, reduces the concentration of readily decomposable chlorine compounds. (See, e.g., DE-A 12 70 036; DD 271 820; U.S. Pat. No. 3,219,678; GB-A 1 080 717; DE-A 22 37 552; U.S. Pat. No. 3,857,871; U.S. Pat. No. 1,458,223; JP 0 727 808 8 A2; JP 0 634 570 7 A2; and GB 1 384 065.) It is mainly the concentration of readily decomposable chlorine compounds that can be determined analytically as acidity which is decreased, whereas short-term heating may suppress the formation of sediments (U.S. Pat. No. 3,274,225).
It is an object of the present invention to provide a new process for purifying organic isocyanates which effectively removes impurities without creating the engineering difficulties experienced with prior art processes.
This and other objects which will be apparent to those skilled in the art are achieved by combining the isocyanate to be purified with an additive corresponding to a specified formula, heating the additive-containing isocyanate for at least 5 minutes at a temperature of from about 100 to about 250xc2x0 C., and at the same time or subsequently the isocyanate being treated is stripped with an inert gas and/or is worked up by distillation and/or extraction.